FIG. 1 illustrates a prior art variable gain amplifier (VGA) having an interpolated attenuator. The VGA of FIG. 1 includes an attenuator network 14 that receives an input signal VIN and provides a series of progressively attenuated versions of the input signal at tap points along the attenuator. A series of input stages 16 selectively couple the attenuated signals to a main amplifier 18 in response to a series of control signals I1 . . . IN from an interpolator 12. The input stages are typically implemented as transconductance (gm) cells having there noninverting (+) inputs connected to corresponding tap points on the attenuator. The maximum gain of the amplifier is set by a network of passive components RA and RB which provide a common feedback signal to the inverting (−) inputs of the gm cells.
If the control signals are implemented as simple on/off signals, the amplifier would behave as a switched-gain amplifier in which the gain changes in discrete steps as different input stages are selected along the length of the interpolator. However, to provide continuously variable gain, the interpolator 12 generates the control signals I1 . . . IN so that adjacent signals are fully or partially “on” to varying degrees, thereby providing a continuum of gain settings between the discrete settings available at the individual taps. That is, the control signals from the interpolator are a series of continuous, overlapping current pulses (in a spatial sense) having a centroid whose location moves along the length of the interpolator as a gain control signal VCTRL is varied so that most of the control signals are nearly zero, but adjacent input stages near the centroid are enabled to some extent. Thus, the VGA of FIG. 1 is referred to as a continuously interpolated VGA.
FIG. 2 illustrates another prior art VGA having continuous interpolation of input stages along an attenuator. The VGA of FIG. 2 is similar to that of FIG. 1 with the following key differences. Rather than having single-ended attenuator, the VGA of FIG. 2 has a fully differential attenuator 22 in which the tap points produce differential pairs of attenuated signals. The feedback signal is no longer applied to the input stages 24. Instead, an active feedback stage including feedback resistors R4 and R5 and active feedback cell 30 are arranged as in an active feedback amplifier (AFA). This frees up the inverting (−) inputs of the input stages, thereby allowing them to operate in a fully differential mode and take full advantage of the differential outputs from the attenuator. In this configuration, the maximum gain is determined by the ratio of the maximum gain of the input section to the gain of the active feedback stage.
Since the input section is no longer directly in a feedback loop, the VGA of FIG. 2 relies on matching and ratios of components in the input section and active feedback stage to provide non-linearity cancellation and other forms of compensation, e.g., temperature, frequency, etc. For example, if the input stages 24 are implemented as gm cells, then the feedback cell 30 is typically implemented as a gm cell that matches the gm cells in the input stages.